Transmission-circuit.



B. W. KENDALL. TRANSMISSION CIRCUIT. APPLICATION men mm: 5. I916.

1 ,1 36m Patented Feb. 13, 1917..

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09m? 727/.- Bw'ion W, Kendall BURTON W. KENDALL, OF NEW YORK, N. Y., ASSIGNOR T WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPQRATION OF NEW YGEK SMTSSION-CIBCUIT.

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Specification of Letters Patent. I Patented} IFQb, J13, liglli,

Application filed June 5, 1916. Serial No. 101,751.

To all whom it may concern:

Be it known that T, BURTON W. KENDALL, a citizen of the United States, residing at New York, in the county of New York and State of New York, have invented certain new and useful Improvements in Transmission-Circuits, of which the following is a full, clear, concise, and exact description.

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This invention relates to transmission circuits, and more particularly to the employment of repeaters or amplifiers in connection with the transmission of currents of telephonic frequencies. Still more particularly, 1t relates to the use of vacuum tube repeaters, such as the audion, on telephone lines.

Its object is to efiect a more perfect balance of the circuits with which such repeaters are employed, whereby a greater degree of amplification may be obtained without danger of locally generated oscillations.

ltis now well understood in the telephone art and allied arts that the degree of amplification allowable in repeater operation depends, among other things, upon the extent to which various parts of the repeater network are balanced. Engineers have, for example, taken great pains to balance the line circuits on either side of a repeater station for impedance, and where two-repeater two-way circuits are involved, to produce in the balancing network or artificial line an accurate copy of the impedance of the line section with which the said network is associated.

Where an accurate balance has proved impossible or uncommercially costly, addi-' tional protective devices, such as wave filters, have been resorted to, to prevent such imperfection inbalance from causing the oscillatory circulation of power known as singing in the repeater.

This invention is concerned with another type of unbalance which has been experienced where repeaters are installed under commercial operating'conditions, and especially where the input circuits of vacuum tube repeaters are connected to telephone lines without the use of'intervening electromagnetic transform'ers.

This invention provides a special form. of filter or connection which is inserted directly in front of the repeater element. lit

the repeater from unbalances in the local wiring, which may arise, for example, from an unequal exposure to ground of the two sides of the repeater leads, and which have been found to cause high frequency singing in the repeater, even though the line impedances have been reproduced in the associated networks with sufficient accuracy to prevent singing at telephonic frequencies.

The invention will be more clearly understood by reference to the accompanying drawings in which Figure 1 represents a simple two-way two-repeater circuit employing vacuum tube repeaters of the'audion type connected to the line circuits in accordance with this invention; and Fig. 2 represents a similar circuit of a more complex nature in which two-stage amplification is employed and-in which wave filters are inserted between the telephone line secing coil 10. Between the point of junction of windings 6 and 7 and the point of junction of windings 8 and 9 is inserted a bridge comprlsing resistances 11 and 12 and capacities 13 and 14, as shown. The middle points of the capacity bridge and the resistance bridge thus formed are connected together and to the cathode element 15 of the vacuum tube repeater 16, the input electrode 17 of this repeater being connected through the polarizing battery 18 to one end of the bridge. The anode 19 of this repeater is'maintained positive with respect to the cathode 15 in the usual manner by means of the battery 20. The output circuit of repeater 16 is connected through the condensers 20 to the primary windings 21 of the transformer 22, whose secondary windings 23, 2t, 25 and 26 are connected in series with the line conductors 27 and 28 of another line section which terminates n its balancing'network 29. Repeater 16 is thus arranged to repeat into line cond ctor jas 27 and 28 amplified currents similar to those arriving over line conductors 3 and 4:. In a similar manner, repeater 30 is arranged to repeat into line conductors 3 and 4: telephonic or other currents arriving over line conductors 27 and 28.

If the input side of the repeater 16 were connected directly across the line, that is, across the resistance bridge 11, 12, a line unbalance would arise from the fact that in any vacuum tube repeater, such as the audion, the filament or cathode member 15 is either directly grounded or has a very large capacity to ground because of its direct connection with the filament heating battery 40 and the output circuit battery 20. By connecting the cathode 15 to the middle point of the resistance bridge, a large part of the danger of unbalancing the line, due to this filament ground, is eliminated. It has been found, however, that where the wires leading to different parts of one repeater circuit or several repeater circuits are formed together in a single local cable, as is necessary in commercial work, capacity effects arise between the different wires, which capacities may cause the development of high-frequency potential differences between any wire and ground. lIt will be evident that any high-frequency potential difference between ground and the bridge point to which the grid 17 is connected, will constitute a potential difference across the resistance 11 which may be suflicient to set up a local circulation of power of high frequency. A

vsmall condenser, such as 13, in parallel with this resistance 11 has been found effective to shunt these high-frequency currents away from the repeater input and to eliminate any danger of singing from this source. A similar condenser is shunted across resistance 12 as well for the sake of symmetry.

Withsome types of telephone lines, as for example, with loaded lines, it is difficult to obtain in the balancing network an accurate copy of the line impedance without resorting to an extremely intricate and expensive structure. In such cases, it has been proposed to include between the repeater element and the telephone line a wave filter or group of filters whose function it is to exclude from the repeater, currents of unde-.

sirable frequencies, or, in other words, to limit the range of frequencies reaching the repeater to those involved in normal telephomc conversation. Such an arrangementis shown in Fig. 2, in which 31 and 32 represent low-frequency' and high-frequency,

filters, respectively, forming in effect a connecting line between the telephone line and the input side of the repeater. The rinciple underlying the design of such lters and their mode of application to the use in question will be found fully described in an application for Letters Patent of G. A.

Campbell, Serial No. 40,057, filed July 15, 1915, so that it is needless to enter into a discussion of these matters here. It may be said, however, that broadly the filter is in the nature of a connecting line having an impedance element or elements in series with the line and an impedance element or elements in shunt of the line, the values of the impedance elements being so proportioned line. The middle points of the condenser bridges, comprising the capacities 34,- are connected ifi this case to the middle point. of the resistance bridge, comprising the resistances 11 and 12, and to the cathode 15. The cathode connection may be carried, as in this case, to the central point of each of the condenser bridges, or it may include only one, or it may be carried to any number of these capacity bridges, depending upon' the degree of refinement required in the balance. It is generally sufficient to include only the terminal condenser bridge,in which case the arrangement becomes substantially that of Fig. 1 except for the inclusion of the wave filters 31 and 32. The circuit of Fig. 2 shows two-stage amplification, the amplifier 1 6, for example, having its output circuit "connected through a potentiometer resistance 35, across one portion of which is connected the input circuit of the second amplifier 36, which, in turn, has its output circuit connected to the windings 21 of the repeating coil 22. llfhe purpose of such an arrangement can be found described in Arnold Patent 1,129,942 of March 2,1915.

What is claimed is:

1. In a telephone repeating system, a telephone line, a bridge across said line, comprising a resistance and a capacity in multiple, and a telephone repeater in shunt of a portion of said bridge.

2. In a telephone repeating system, apair of line conductors, a capacity bridge and a resistance bridge between saidline conductors, and a telephone repeater having one,

cathode connected to the middle point of 130 said bridge and its input electrode connected to one terminal of said bridge.

4. In a telephone repeating system, a telephone line, a bridge across said line comprising a capacity and resistance in multiple, a vacuum tube repeater, and a connection between the middle point of said bridge and the cathode of said repeater.

5. In a telephone repeating system, a telephone line, a capacity bridge and a. resistance bridge across said line, a vacuum tube repeater, anda connection between the middle points of said bridges and the cathode of said repeater.

6. In a telephone repeating system, a pair of line conductors, a capacity bridge and a resistance bridge between said conductors,

"a thermionic repeater having its cathode connected to the middle point of said bridges and having its input electrode connected to one of said line conductors.

7. In a telephone repeating system, atelephone line, a telephone repeater, a connecting line between said telephone line and said repeater, a plurality of condenser bridges across said connecting line, and a connection between the middle points of said condenser bridges and one terminal of said repeater.

8. In a telephone repeating system, a telephone line, a vacuum tube repeater, a connecting line between said telephone line and said repeater, a plurality of condenser bridges across said connecting line, and a connection between the middle point of one of said bridges and the cathode element of said repeater.

9. In a telephone repeating system, a telephone line, a vacuum tube, repeater, a connecting line between said telephone line and said repeater, a plurality of condenser bridges and a resistance bridge across said connecting line, and a connection between the middle points of said bridges and the cathode element of said repeater.

10. In a telephone repeating system, a telephone line, a vacuum tube repeater, a connecting line between said telephone line and said repeater, a plurality of condenser bridges and a resistance bridge. across said connecting line, and a connection between the middle points of said resistance bridge and one of said condenser bridges'and the cathode of said repeater.

In witness whereof, I hereunto subscribe my name this 3rd day of June A. D., 1916.

BURTON w. KENDALL. V 

